The proposed research is a biophysical study of biological lipid membranes and membrane-active peptides. It relies primarily on methods of infrared (IR) spectroscopy and lipid film balance techniques. The research aims to : (1) determine the conformation and orientation of membrane-bound cecropin A, (2) characterize the process of membrane recognition by cecropin A and sarcotoxin 1A, (3) verify an oligomeric model for membrane recognition by the influenza fusion peptide, and (4) develop essential software and instrumentation. This research will improve the understanding of the fundamental biophysical processes underlying peptide-lipid interactions in which peptides undergo programmed conformational changes upon encountering a lipid membrane. These processes are critical to understanding viral fusion--an important pathogenic mechanism in human infections such as influenza, measles, and HIV--and to understanding the role of naturally occurring antimicrobial peptides in host defense against infection.